The Move Deleter is a person who deletes all moves, even HMs. He always makes the joke and I quote: " Who am I again? Oh yeah I'm the Move Deleter!" . To have a Pokémon remember a move, just give the Move Rememberer a Heart Scale which is accessed from the Underground.
In pokemon dimond the move deleter is found in the Canalave city and the move rememberer in Pastoria City. In Heart gold it is in Blackthorn city. You can get heart scales as a prize in the pokeathlon Dome near the national park. (Only on wednesdays)